In semiconductor manufacturing, silicon on insulator (SOI) structures are becoming increasingly attractive for low power, high performance complementary metal oxide semiconductor (CMOS) applications. A variety of SOI structures are presently available in the semiconductor industry. These SOI structures fall into two categories: SIMOX (separation by implantation of oxygen) and bonded SOI. There are three kinds of commercially available SIMOX materials: standard high dose (1.6-1.8.times.10.sup.18 cm.sup.-2), medium dose (0.8-1.2.times.10.sup.18 cm.sup.-2) and low dose (0.3-0.7.times.10.sup.18 cm.sup.-2).
Similarly, there are three kinds of commercially bonded SOI structures: bonded and plasma thinned (acuthin), bonded and smartcut (Unibond) and bonded and etch back (Sibond). In each of the foregoing bonding techniques, it is preferred to have the bond seam away from the active Si region to minimize impact of bonding imperfections on devices and circuits built on the Si.
The temperature employed in prior art SOI bonding techniques is typically limited to temperatures below 1150.degree. C. This is partly due to the availability of furnaces in the silicon fabrication line which are designed to operate at temperatures less than 1200.degree. C. More importantly, however, is that previous findings have been reported which indicate that bonding perfection between Si and SiO.sub.2 or two SiO.sub.2 surfaces can be achieved at temperatures less than 1100.degree. C. The characterization techniques employed in the prior art to determine the quality of the bonding process are conventional optical, infrared or acoustic microscopy, X-ray topography and TEM. Although these techniques can detect gross bonding defects, such as voids, they cannot determine the physical (roughness) and mechanical (bonding strength) properties of the interface.
U.S. Pat. No. 4,710,030 to Tauc, et al. describes an optical stress pulse generation and detection system for non-destructively measuring the physical properties of a sample. Specifically, the '030 patent utilizes a pump beam having short duration radiation pulses having an intensity and at least one wavelength selected to non-destructively generate a stress pulse in a sample and directs the non-destructive pump beam to a surface of a sample to generate the stress pulse. The '030 patent does not teach that the same can be employed in detecting the bonding imperfections of previously bonded SOI structures.
The presence of bonding imperfections in SOI structures can create a fixed charge either in the buried oxide region or in the silicon/buried oxide interfacial region. The presence of such a charge is undesirable because it can shift the threshold voltage of devices fabricated on this structure.
In view of the drawbacks mentioned hereinabove, there remains a need for providing a method which can be used to improve the bonding of previously bonded SOI structures so that the newly bonded SOI structure does not exhibit any bonding imperfections at the interface between the bonded silicon material and the insulating material.